5.7 HIGHWAY PROBLEMS RELATED TO GRADATION CHANGES
Gradation problems at highway crossings include changes in the vertical dimension such as
aggradation and degradation; however, lateral erosion problems often occur as a consequence
of these changes. The highway problem most associated with aggradation is reduction of flow
area, which increases backwater effects upstream of bridges and culverts. Problems
associated with degradation are undermining of footings, pile bents, abutments, cutoff walls,
and other flow-control or crossing structures. Degradation has also been found to undermine
bank protection resulting in the instability of channel banks and increasing debris problems. A
common problem associated with lateral erosion is bank slumping, which undermines
abutments and piers located near the bank line. Another very common problem arises when
meandering streams, migrate laterally and encroach upon roadways.
Causes of gradation changes that have an impact on highway crossings can be grouped into
two basic categories: (1) the result of human activities; and (2) natural causes or factors. An
analysis of case histories indicates that very few gradation changes were due to natural factors.
Some gradation changes should perhaps be classified as being caused by a combination of
both natural and human-induced factors. However, their number is so small that a separate
category is not warranted. Because human activities dominate the causes for gradation
problems, they will be discussed first.
5.7.1 Changes Due to Human Activities
Human activities are literally changing the face of the Earth and generate accelerated erosion
from watersheds. Some activities have had far-reaching consequences on streams and have
caused, or contributed to aggradation and degradation problems at bridges. Construction of a
bridge and approach embankments may also have consequences, but they are unlikely to be
far-reaching. Human activities were found to be the major cause of streambed elevation
changes. Because accelerated erosion is associated with human activities, it is often possible
to anticipate many impacts on bank stability and provide adequate bank protection in advance.
From an analysis of case histories, human activities resulting in gradation problems can be
grouped into the following categories: (1) channel alterations; (2) land use changes; (3)
streambed mining/excavation; and (4) dams and reservoirs.
Channel Alterations. Straightening, dredging, clearing and snagging, artificial constrictions, and
other alterations of natural channels are the major causes of streambed elevation changes.
Channel straightening is the dominant activity. Examples of channel response to straightening
are presented in Keefer et al. (1980).
Many of the straightened channels have degraded, and degradation is usually accompanied by
widening of the channel, unstable banks and serious debris problems. The degradation is
attributed to an increase in channel slope that results from shortening of channel length. The
increase in channel slope increases the velocity and the shear stress on the bed. As a result,
the channel bed degrades until the bed becomes armored or the channel widens and begins to
meander to reduce the channel slope back to an equilibrium, or stable condition. There is some
evidence that degradation, if it is to occur as a consequence of channel alteration, will be most
rapid during a period shortly following the alteration and will thereafter occur at a decreasing
rate.
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